Another approach to accomplish the unambiguous measurement of blood volume flow is described in an Article by Hottinger and Meindl entitled "An Ultrasonic Technique for Unambiguous Measurement of Blood Volume Flow" published in 1974 and given at the IEEE Ultrasonic Symposium Proceedings and in the U.S. Pat. to Hottinger No. 4,067,236, both incorporated herein by reference. These references disclose a flow measurement technique that provides for multiplication of a velocity component perpendicular to a sample plane by the cross-sectional area of the sample plane. Three methods based on this principle are disclosed and in all three cases, uniform isonofication of the sample cross-section is required and must be used to measure the velocity component normal to the cross-section. The projected or effective cross-sectional area is measured by one of three methods: (1) a two-dimensional array Doppler C-scan; (2) a linear array Doppler B-scan; and (3) measurement of area in terms of Doppler signal power. The third method utilizes an inner transducer to compensate for attenuation losses and scattering losses of an annular outer transducer. A signal from a sample region which lies totally within the vessel is obtained by the inner transducer. This sample region lies totally within the region sampled by the combination of the outer and inner transducers. The projected lumen area is a ratio of signal powers from the two transducers. Unfortunately, this method requires knowledge of the sample dimensions of the inner transducer and the relative gains of the two transducers and their associated circuitry. This third method appears to be very similar to that disclosed in the U.S. Pat. to Hassler No. 3,977,247, incorporated herein by reference. This third method has further disadvantages in that the circuitry is more complicated (and hence more expensive) since two transducers and two separate electrical systems for processing the information are required. In addition, because two separate electrical systems are used, the systems must be electrically matched so that the same signal will be identically processed by both systems in order to eliminate induced errors, or the gain coefficients of each system must be known so that additional compensation circuitry can be employed.
In addition to the foregoing United States Patents mentioned hereinabove, two other patents showing similar systems and methods are depicted in U.S. Pats. to Shaw, No. 3,498,290 and to Hansen No. 3,987,673. All of these references are incorporated herein by reference to show, at least, the conventional methods and circuitry to accomplish those methods.